Spark-type and compression-type internal combustion engines generate several forms of nitrogen oxide (NOx) gasses as byproducts of the fuel combustion process. NOx gasses may be present in the engine exhaust stream in various forms, including nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). To reduce the levels of the various NOx gasses present in vehicle tailpipe emissions, modern vehicles may include a selective catalytic reduction (SCR) device.
Within the exhaust system, a suitable reductant such as ammonia or urea is added at a precisely controlled rate to the NOx-containing exhaust stream, and passed over the SCR device. The catalytic action of the SCR device converts the NOx gasses into nitrogen and water. NOx sensors are typically positioned within the exhaust stream to closely monitor NOx levels and SCR efficiency. The performance of these sensors may be evaluated using appropriate sensor diagnostic software.